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Component internal rotation malrotation a factor in pain after TKA
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This study, one of the largest CT rotational profiling investigations to compare patients with pain following total knee arthroplasty with a matched control group, showed significant internal rotation of components in patients with pain.
The study also indicated that 45% of those with pain have greater than 3° internal rotation component mismatch.
“We’ve identified that internal rotation of the femoral and tibial components individually, as well as combined, were factors in painful total knee arthroplasty,” Stuart W. Bell, MBChB, MRCP, MRCS, said in his presentation at the 12th EFORT Congress 2011. “We also found that internal rotation mismatch was a factor, but no absolute figures of rotation were identified as unacceptable, and there was a broad range of rotation found in both cohorts indicating that other factors are clearly at play.”
Measuring component rotation
Bell and his Scottish colleagues compared 56 patients who experienced unexplained pain after total knee arthroplasty (TKA) with a control group of 59 patients, excluding patients with obvious causes of pain or infection. The researchers then matched the groups for age, diagnosis, gender, preoperative Oxford Knee scores and body mass index.
Using the standard Berger protocol, which quantitatively assesses prosthetic malrotation on CT scans, investigators measured rotation of the femoral and tibial components. From these values researchers calculated the combined component rotation and component mismatch. Combined component rotation was calculated by adding the femoral and tibial rotation together and they calculated component rotation mismatch by subtracting femoral rotation from tibial rotation values.
Mechanical alignment differences
“We found a significant difference in postoperative mechanical alignment as well as the postoperative Oxford scores and Visual Analog scores between the two groups, and differences in the scores can be expected due to our inclusion criteria for the two cohorts,” Bell said.
The team also found statistically significant differences in internal rotation between the groups regarding the tibial component, femoral component and combined rotation.
For tibial component, the study group had 3.46° internal rotation compared with 2.5° external rotation for the control group (P=0.001). For femoral components, the team calculated 2.3° internal rotation for the study group compared with external rotation of 0.36° in the control group (P=0.002).
For combined component rotation, the researchers found 7.08° internal rotation in the study group vs. 2.85° external rotation in the control group (P=0.001). “When looking at the component rotational mismatch, we looked for a portion of patients in each cohort with greater than 3· of internal rotation,” Bell said. “ … There were 45% in the painful group with greater than 3° of internal rotation mismatch.”
Bell described the potential weaknesses in the study as the “accuracy of the accepted anatomical mean values used, the possibility of anatomical variations within the population, and the accuracy of the measurements of the angles.” – by Renee Blisard
Reference:
- Bell S, Young P, Drury C, et al. Internally rotated components in the painful total knee arthroplasty patient: A matched patient cohort study using computerized tomography. Paper #1959. Presented at the 12th EFORT Congress 2011. June 1-4. Copenhagen, Denmark.
- Stuart W. Bell, MBChB, MRCP, MRCS, can be reached at Glasgow Royal Infirmary, 84 Castle St., Glasgow G40SF, United Kingdom; +44-141-221-4000; email: drstuartbell@gmail.com.
- Disclosure: Bell has no relevant financial disclosures.